How to Use the std::map::find Function in C++

1. Use the std::map::find Function to Find the Element With a Given Key Value in C++
2. Use the contains Member Function to Check if the Given Element Exists in a Map in C++

This article explains how to utilize the std::map::find function and some of its alternatives in C++.

Use the std::map::find Function to Find the Element With a Given Key Value in C++

The std::map object is one of the associative containers in the C++ standard template library, and it implements a sorted data structure, storing key values. Note that keys are unique in the std::map container. Thus, if new elements are inserted with the existing keys, the operation does not have any effect. Still, some special member functions in the std::map class can assign new values to the existing pairs if the keys are matched (e.g. insert_or_assign function).

The advantages of the std::map container include fast search, insertion, and removal operations, which can be conducted in logarithmic time. The search operation is provided by the find member, which accepts a reference to a key. If the given key is found in the std::map object, the iterator to the corresponding element is returned. On the other hand, suppose the given key is not found in the container, the past-the-end iterator (map::end()) is returned.

The following code snippet demonstrates a simple usage scenario where a map container of string pairs is initialized with arbitrary values, and then the key with the "h" value is searched. Consequently, we process the returned iterator with the if-else statement to either print the element pair or output a message that the given key was not found.

#include <iostream>
#include <map>

using std::cin;
using std::cout;
using std::endl;
using std::map;
using std::string;

int main() {
  std::map<string, string> m1 = {
      {"h", "htop"}, {"k", "ktop"}, {"t", "ttop"},
      {"r", "rtop"}, {"w", "wtop"}, {"p", "ptop"},
  };

  string key = "h";

  auto item = m1.find(key);
  if (item != m1.end()) {
    cout << "Key exists!  -  {" << item->first << ";" << item->second << "}\n";
  } else {
    cout << "Key does not exist!" << endl;
  }

  return EXIT_SUCCESS;
}

Output:

Key exists!  -  {h;htop}

Use the contains Member Function to Check if the Given Element Exists in a Map in C++

If the user needs to confirm if the pair with the given value exists in the map object, one can utilize the member function contains. The function has been part of the std::map container since the C++20 version, so you should know the compiler version to run the following code snippet. The contains function takes a reference to the key and returns the bool value true if it’s found. The time complexity of this member function is also logarithmic.

#include <iostream>
#include <map>

using std::cin;
using std::cout;
using std::endl;
using std::map;
using std::string;

int main() {
  std::map<string, string> m1 = {
      {"h", "htop"}, {"k", "ktop"}, {"t", "ttop"},
      {"r", "rtop"}, {"w", "wtop"}, {"p", "ptop"},
  };

  string key = "h";

  if (m1.contains(key)) {
    cout << "Key Exists!" << endl;
  } else {
    cout << "Key does not exist!" << endl;
  }

  return EXIT_SUCCESS;
}

Output:

Key Exists!

Alternatively, one can use the cout member function to check if the element pair with the given key exists in the map. Generally, the cout function is utilized to retrieve the number of elements with the given key, but since the std::map stores only unique key values, the process returns 1 if the element is found. Otherwise, a zero value is returned to indicate that no element was found.

#include <iostream>
#include <map>

using std::cin;
using std::cout;
using std::endl;
using std::map;
using std::string;

int main() {
  std::map<string, string> m1 = {
      {"h", "htop"}, {"k", "ktop"}, {"t", "ttop"},
      {"r", "rtop"}, {"w", "wtop"}, {"p", "ptop"},
  };

  string key = "h";

  if (m1.count(key)) {
    cout << "Key Exists!" << endl;
  } else {
    cout << "Key does not exist!" << endl;
  }

  return EXIT_SUCCESS;
}

Output:

Key Exists!